<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
</head>

<body>
    <!-- 输入框 1-->
    <input id="a1" />
    <!-- 输入框旁的+-*/ -->
    <select name="" id="js">
      <option value="+">+</option>
      <option value="-">-</option>
      <option value="*">*</option>
      <option value="/">/</option>
    </select>
    <!-- 输入框2 -->
    <input id="a2" />
    <!-- 计算按钮 -->
    <button id="a3">计算</button>
    <!-- 结果框 -->
    结果：
    <input type="text" name="" id="res" />
    <script>
        var aEle = document.getElementById("a3");
        var a1Ele = document.gteElementById("a1");
        var a2Ele = document.gteElementById("a2");
        var resEle = document.getElementById('res');
        var jsEle = document.getElementById("js");
        aEle.onclick = function() {
            var a1 = parseInt(a1Ele.value)
            var a2 = parseInt(a2Ele.value)
            var calc =
        };
    </script>
</body>

</html>